Lithium niobate optical modulator

ABSTRACT

An optical modulator comprises a Z-cut lithium niobate substrate ( 21 ) on which is formed a Mach-Zehnder interferometer having two generally parallel waveguides ( 23, 25 ) lying beneath a buffer layer of dielectric material ( 27 ). First and second ground electrodes ( 29, 33 ) and a hot electrode ( 31 ) are disposed on the buffer layer ( 27 ), the first and second ground electrodes ( 29, 33 ) being spaced either side of the hot electrode ( 31 ), the hot electrode ( 31 ) and the first ground electrode ( 29 ) being proximate to at least apart of the respective waveguides ( 25, 23 ). The electrode structure is unsymmetrical in that (a) the hot electrode and the first ground electrode each have a width substantially less than that of the second ground electrode and or (b) the spacing between the first ground and hot electrodes is different from the spacing between the second ground and hot electrodes. whereby a range of chirp values can be obtained. When the spacing (G 1 ) between the first ground and hot electrodes ( 29, 31 ) is smaller than the spacing (G 2 ) between the second ground and hot electrodes ( 33, 31 ), and preferably the hot and first ground electrodes have a width not exceeding 15 μm, the modulator is capable of operation at frequencies above 10 GHz, possibly up to around 40 GHz.

This invention relates to lithium niobate optical modulators. Inparticular, the invention relates to lithium niobate optical modulatorswith electrode structures enabling chirp parameters to be chosen andcontrolled; some forms of the invention also achieve operation at highfrequencies (bit rates) of up to 40 GHz.

As the demand for telecommunications services and bandwidth has boomed,the need for, and advantages of, external modulation in fibre-optictransmission systems has been firmly established. Lithium niobate istoday one of the most important dielectric materials in the field ofintegrated optics, both for research and for technological applications.This importance is due to the strong correlation between the opticalproperties of the crystal, its refractive index, and the application ofvarious kinds of external fields; namely electric fields (electro-opticeffect), sound waves (acousto-optic effect) and electromagnetic waves.Lithium niobate external modulators provide both the required bandwidthand a means for mining the effects of dispersion that limit systemperformance.

Almost all lithium niobate optical modulators are travelling wavedevices, in which the optical waveguide comprises a Mach-Zehnderinterferometer (MZI). High speed, broad bandwidth optical modulators aremade by constructing a particular electrode structure on the bufferlayer of the MZI modulator, which prevents light propagating through thewaveguide path from being absorbed by the electrode metal. MZImodulators usually operate with a push-pull electrode structure, so thatfields of opposite polarity operate on each arm of the waveguide. Thesefields serve to change the index of the electro-optic lithium niobate,which in turn alters the phase of the light travelling in eachwaveguide, and thus allows operation of the interferometer. The opticalphase or amplitude modulation results from an interaction between theoptical wave in the optical waveguide and the microwave wave guided bythe electrode structure. Meantime, however, the refractive index of thelithium niobate also changes in response to exposure to light, includingthe light being modulated, producing changes that can be interpreted asphase modulation or as a change of frequency spectrum within the digitalpulses, and which are known as “chirp”. In an idealised system operatingwith perfectly monochromatic light, chirp would be a defect to beminimised, but in a real system with a finite range of opticalwavelengths, it is possible to exploit chirp to counter chromaticdispersion occurring elsewhere in the system, and so it is desirable tobe able to make modulators with pre-selected chirp values, preferablyover a substantial range.

Lithium niobate MZI devices have the potential for very broadbandoperation, but they are limited by, inter alia, mismatches betweenoptical and microwave effective refractive indices (and hence mismatchesbetween the velocities of the electrical and optical signals), electrodeelectrical losses, electrode impedance and drive voltage. Specifically,a velocity mismatch between the velocities of the electrical and opticalsignals together with electrical losses strongly curb modulatorelectro-optical response; high electrode impedance is needed to preventreflections when the modulator is connected to a signal electricaldriver; and low driving voltages are a prerequisite. Introducing a verythin dielectric layer between the MZI structure and the electrodes canprovide velocity matching between the electrical and optical fields, lowelectrical losses and high impedances, but requires higher drivingvoltages. Modulators made accordingly based on both X-cut and Z-cutlithium niobate substrates perform well up to 10 GHz (the electro-opticeffect in lithium niobate is anisotropic and highest in the Z directionof the crystal's unit cell; to maximise the electric field in thatdirection and so the depth of modulation, waveguides in a Z-cut surfacewill normally be located directly below electrodes, whereas those in anX-cut surface will normally be located in the gap between twoelectrodes). For operation at frequencies in excess of 10 GHz using thesame driving voltage and microwave refractive index values, Z-cutlithium niobate substrates enable lower electrical losses and higherimpedances.

Typical electrode structures used for Z-cut lithium niobate MZImodulators include coplanar strip structures, as shown in FIG. 1, havingrespective electrodes 1, 3 disposed on the buffer layer 5 directly aboveeach of two parallel waveguides 7, 9 forming the MZI and having equalwidths approximately the same as those of the waveguides. Suchstructures provide high impedance, but also high electrical losses.Asymmetric coplanar strip electrode structures, as shown in FIG. 2, aresimilar to the coplanar strip structures of FIG. 1 except that theground electrode 3 is much wider than the hot electrode 1. Thesestructures present problems with electrical-optical signal velocitymatching, which may be overcome by use of very thick gold electrodes(>30 μm), though such thicknesses are difficult to obtain using standardprocesses. Such structures also cannot achieve very low electricallosses suitable for high frequency applications, and their structuredoes not permit easy electrical connections to be made within a package.

Coplanar waveguide structures, as shown in FIG. 3, are similar toasymmetric coplanar strip structures of FIG. 2 with a second wide groundelectrode 4 spaced symmetrically on the other side of the narrow hotelectrode 1. These structures can provide good electrical losscharacteristics, but to achieve optical-microwave effective refractiveindex matching and high impedance requires an increase in buffer layerthickness which then requires higher driving voltages. One way toachieve optical-microwave effective refractive index matching and highimpedance using a thinner buffer layer would be to use narrow groundelectrodes. However, narrow electrodes suffer from very high electricallosses.

There remains a need for a Z-cut lithium niobate optical modulatordesign which is capable of operating at frequencies in excess of 10 GHzwith matching optical and microwave effective refractive indices, lowdrive voltage and electrode electrical losses, and high electrodeimpedance; some forms of the present invention solve this problem, aswell as providing choice of chirp values at frequencies at present inuse.

According to the invention, there is provided an optical modulatorcomprising a Z-cut lithium niobate substrate on which is formed aMach-Zehnder interferometer having two generally parallel waveguideslying beneath a buffer layer of dielectric material. First and secondground electrodes and a hot electrode are disposed on the buffer layer,the first and second ground electrodes being spaced either side of thehot electrode, the hot electrode and the first ground electrode beingproximate to at least a part of the respective waveguides. The inventionis characterized by an asymmetrical electrode structure in which

-   -   (a) the hot electrode and the first ground electrode each have a        width substantially less than that of the second ground        electrode and/or    -   (b) the spacing between the first ground and hot electrodes is        different from the spacing between the second ground and hot        electrodes.

The hot electrode will normally have a width approximately equal to thewidth of the waveguide beneath it, and when option (a) applies, this ispreferably also true of the first ground electrode; in this case,therefore, the widths of the hot electrode and the first groundelectrode will usually be substantially equal and not normally.exceeding 15 μm. Preferably they will each have a width in the range5-15 μm. The second ground electrode preferably has a width at leastfive times greater than that of the hot electrode (and usually the firstground electrode); more especially preferably at least ten times wider,in this context considered an “infinite” width.

By adjustment of the two spacings between the hot electrode and therespective ground electrode, and/or of the widths of the threeelectrodes, a useful range of chirp values can be obtained, as will beillustrated below.

By use of a smaller spacing between the first ground electrode and thehot electrode than between the second ground electrode and the hotelectrode, and preferably a first ground electrode with a width notexceeding 15 μm, the inventors have been able to obtain a good matchbetween the microwave and optical effective refractive indices, somaintaining low electrical losses and good impedance characteristics.This action does not affect the driving voltage since neither the bufferlayer thickness nor the spacing between the first ground and hotelectrodes are adversely changed. Furthermore, by appropriate selectionof geometrical parameters, it remains possible to provide a modulatorstructure with a residual chirp value close to zero (as an absolutevalue, irrespective of sign; as is understood in the art, only themagnitude of the chirp value is significant, because its sign can bereversed by setting bias to chose operation on a part of the sinusoidaloptical output intensity/driving voltage characteristic of the devicethat has negative or positive slope).

Preferably, the spacing between the first ground and hot electrodes isbetween 10 and 30 μm and the spacing between the second ground and hotelectrodes is larger, between 20 and 80 μm.

Use of a (second) wide ground electrode in some preferred forms of theinvention ensures low electrical loss, while the combination of thiswith a (first) narrow ground electrode ensures low driving voltage andhigh impedance. The narrow ground electrode further serves to reduce themicrowave effective refractive index relative to the optical refractiveindex.

Preferably, the dielectric material comprises silicon dioxide with athickness between 0:4 and 1.5 μm, and the electrodes comprise goldhaving a thickness between 15 and 50 μm; thicknesses up to 30 μm areeasier'to obtain with present electroplating techniques.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from the description or recognizedby practicing the invention as described in the written description andclaims hereof, as well as the appended drawings. It is to be understoodthat both the foregoing description and the following detaileddescription are merely exemplary of the invention, and are intended toprovide an overview or framework to understanding the nature andcharacter of the invention as it is claimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate one or moreembodiments of the invention, and together with the description serve toexplain the principles and operation of the invention.

FIGS. 1 to 3 are sections through respective conventional coplanarstrip, asymmetric coplanar strip and coplanar waveguide Z-cut modulatorstructures;

FIG. 4 and 5 are section and plan views of a preferred modulatorstructure according to the invention;

FIGS. 6 and 7 are section and plan views of a second modulator structureaccording to the invention;

FIGS. 8 to 11 are graphs showing the dependence of drive voltage,electrical loss, microwave index and impedance against ground electrodewidths; and

FIG. 12 is a graph showing the dependence of residual chirp on electrodespacing in the modulator of FIGS. 4 and 5.

A first preferred asymmetric coplanar waveguide modulator structureaccording to the invention, shown in FIGS. 4 and 5, comprises a Z-cutlithium niobate substrate 21 with a thickness of around 500 μmpresenting an optical MZI on its surface. The MZI comprises a midsection having two generally parallel waveguides 23, 25 which convergeat either end into single input and output waveguides 22, 26. Theparallel waveguides are formed in the plane of the surface of thesubstrate by a standard titanium diffusion process well known to thoseskilled in the art of modulator design. The axes of the parallelwaveguides 23, 25 are 25 μm apart and they have a depth of around 6 μmand a width of around 10 μm. A buffer layer 27 of silicon dioxide havinga dielectric constant of around 4 and a thickness of 0.65 μm is grown bymeans of a conventional electron beam evaporation process directly onthe surface of the lithium niobate substrate in which the waveguides 23,25 are formed. Three parallel gold electrodes 29, 31, 33 are grown onthe buffer layer 27 to a thickness of 27 μm. Two of the electrodes 29,31 are disposed directly above the parallel waveguides 23, 25 with awidth of around 7 μm. and a length extending to just short of the endsof the parallel sections of the waveguides 23, 25. The third electrode33 is spaced 50 μm from the middle electrode 31 with a width of 150 μmand the same length as the other electrodes.

Using the middle electrode 31 as the hot electrode, the narrow electrode29 to the side of the hot electrode as a first ground electrode and thewide electrode 33 on the other side of the hot electrode 31 as a secondground electrode, the asymmetric coplanar waveguide modulator structuredescribed has a chirp (alpha parameter modulus) value of about 0.05 (asfurther discussed below) and functions well at frequencies up to andincluding 40 GHz. The narrow first ground electrode 29 ensures thatdriving voltages may be kept low while maintaining high impedance. FIG.8 shows the effect of increasing ground electrode widths on drivevoltage, and FIG. 11 shows the effect of increasing ground electrodewidths on impedance for a conventional symmetric coplanar waveguidestructure. The narrow first ground electrode 29 also serves to reducethe microwave effective refractive index, enabling it to be brought downto match the optical refractive index, while still widening the space G2between the second ground electrode 33 and the hot electrode 31 tobenefit from low electrical loss and high impedance. Thus, the bufferlayer may be kept within parameters for maintaining low drivingvoltages. The effect of increasing ground electrode widths on thevelocity matching parameter is demonstrated graphically in FIG. 10. Thewide second ground electrode 33 on the other hand ensures low electricalloss, as demonstrated in FIG. 9; also such graphs are related to aconventional symmetric coplanar waveguide structure.

Table 1 provides a comparison between critical geometrical andperformance parameters of the asymmetric coplanar waveguide modulatorstructure as shown in FIGS. 4 and 5 (“invention”) and two coplanarwaveguide modulator structures as shown in FIG. 3, one with wide groundelectrodes (CPW “inf”) and one with narrow ground electrodes (CPW“fin”): TABLE 1 W G1 G2 FW1 FW2 t Parameter (μm) (μm) (μm) (μm) (μm)(μm) Invention 7 18 50 7 Inf 27 CPW “inf” 7 18 18 Inf Inf 27 CPW “fin” 718 18 7 7 27 τ Z₀ EL(dB/cm) V_(π)*L |α| (chirp Parameter (μm) N_(m) (Ω)at 40 GHz (V*cm) parameter) Invention 0.65 2.15 50 2.45 11 0.05 CPW“inf” 0.65 2.21 34 2.5 11 0.66 CPW “fin” 0.65 2.11 41 3 10.7 0.32where the geometrical parameters are as shown in FIG. 4, N_(m) is themicrowave effective refractive index, Z₀ is the impedance, EL is theelectrical loss figure, and V_(π)L is the drive voltage*electrode lengthproduct

As can be seen, CPW “inf” gives a high microwave effective refractiveindex figure (2.21), while CPW “fin” has large electrical losses (3dB/cm). However, the structure according to the invention displays theadvantageous electrical loss figure of CPW “inf” and the velocitymatching characteristics of CPW “fin” with high impedance and the samedrive voltage.

In external modulators, chirp arises from phase modulation superposed onthe modulated optical signal due to unequal modulation applied to therespective arms of the interferometer. If the integrals of thecorresponding electrical and optical fields for the two arms are notequal, a residual chirp arises. For X-cut modulators, the electrodestructure is generally symmetrical with the optical waveguidespositioned beneath the buffer layer but symmetrically between theelectrodes so the relative driving voltages are the same. Provided thereare no spurious effects, then no residual chirp is expected.

For conventional Z-cut modulator structures such as that shown in FIG.3, the combined electrical and optical structure cross sections areasymmetric (though the electrode structure and the optical structure,considered separately, are each symmetrical), with the opticalwaveguides placed beneath the hot electrode and one of the groundelectrodes. Typical expected driving voltage*electrode length productsfor such a structure would be 10-15V*cm and 70-100V*cm for therespective arms, with the integral of the fields relating to the hotelectrode arm being much higher than that relating to the other arm,leading to a fixed residual chirp. However, with the structure of theembodiment of the invention shown in FIGS. 4 and 5, the narrow groundelectrode 29 over the waveguide 23 gives rise to driving voltages muchcloser to those exhibited by the coplanar strip structure of FIG. 1,typically of the order of 15-25V*cm for the hot electrode arm and20-30V*cm for the ground electrode arm. Such close driving voltages leadto lower residual chirp than could be expected from the structures ofFIGS. 3 and 2. FIG. 12 shows the effect that varying the spaces G1 andG2 between the hot electrode and the two ground electrodes in thestructure of FIGS. 4 and 5 is calculated to have on residual chirp. Fora fixed space G1, increasing G2 has the general effect of decreasingresidual chirp. Conversely, for a fixed space G2, increasing G1 has thegeneral effect of increasing residual chirp. In particular, by properlymanaging the geometrical parameters of the structure in FIG. 4, residualchirp values (|α|) lower than 0.1 (usually considered to be a zero-chirpmodulator) can be achieved.

In order to benefit fully from the respective advantages of the narrowand wide ground electrodes as described above requires that the wideelectrode be at least ten times wider than the narrow electrode, and thenarrow ground electrode to be preferably the same width as the hotelectrode.

Table 2 below summarises a series of modulator designs, all inaccordance with FIG. 4 and 5, designed for operation at frequencies atpresent in operation (say 10 GHz) and for various different chirpvalues. Throughout the series, the hot electrode and first groundelectrode each had a width of 8 μm and the second ground electrode atleast 80 μm (“infinite”), and the electrodes were 10 mm long and about20-25 μm thick.

In the table, d is the distance between the axes of the two waveguides,G1 the spacing between the hot electrode and the first ground electrode,G2 the spacing between the hot electrode and the second groundelectrode, and τ the thickness of the buffer layer, all as shown in FIG.4. V_(π) is the drive voltage (which, since the electrode length is 10mm, is numerically equal to the driving voltage *electrode lengthproduct). Chirp is tabulated as modulus (that is without its sign.)TABLE 2 d G1 G2 T V_(π) Chirp (|α|) 22 14 6 0.55 9.63 0.45 10 0.55 9.780.29 14 0.55 9.85 0.21 18 0.55 10.03 0.16 26 0.6 10.45 0.10 34 0.6 10.500.07 42 0.6 10.53 0.05 26 18 6 0.55 9.80 0.51 10 0.55 10.10 0.37 14 0.5510.20 0.27 22 0.55 10.40 0.17 30 0.65 11.60 0.12 38 0.65 11.13 0.09 460.65 11.13 0.06 32 24 8 0.6 10.53 0.52 12 0.6 10.68 0.38 16 0.6 10.950.29 24 0.7 11.75 0.19 32 0.75 12.10 0.14 40 0.75 12.10 0.10 48 0.7512.15 0.07 43 35 10 0.7 11.83 0.54 16 0.7 12.08 0.39 22 0.7 12.20 0.2928 0.7 12.33 0.22 35 0.8 13.10 0.17 42 0.8 13.40 0.13 50 0.8 13.20 0.10

FIGS. 6 and 7 show an alternative asymmetric coplanar wavegaidestructure according to the invention in which the second groundelectrode is the same width as the other two electrodes but the spacingof the electrodes is unequal. This structure achieves low drivingvoltages and high impedance, but it does suffer from higher electricalloss than the structure of FIGS. 4 and 5.

Any discussion of the background to the invention herein is included toexplain the context of the invention. Where any document or informationis referred to as “known ” it is admitted only that it was known to atleast one member of the public somewhere prior to the date of thisapplication. Unless the content of the reference otherwise clearlyindicates, no admission is made that such knowledge was available to thepublic or to experts in the art to which the invention relates in anyparticular country (whether a member-state of the PCT or not), nor thatit was known or disclosed before the invention was made or prior to anyclaimed date. Further, no admission is made that any document orinformation forms part of the common general knowledge of the art eitheron a world-wide basis or in any country and it is not believed that anyof it does so.

1. An optical modulator comprising a Z-cut lithium niobate substrate onwhich is formed a Mach-Zehnder interferometer having two generallyparallel waveguides lying beneath a buffer layer of dielectric material,and first and second ground electrodes and a hot electrode disposed onthe buffer layer, the first and second ground electrodes being spacedeither side of the hot electrode, the hot electrode and the first groundelectrode being proximate to at least a part of the respectivewaveguides, characterized by an asymmetrical electrode structure inwhich: (a) the hot electrode and the first ground electrode each have awidth substantially less than that of the second ground electrodesand/or (b) the spacing between the first ground and hot electrodes isdifferent from the spacing between the second ground and hot electrodes.2. The optical modulator according to claim 1, wherein the hot electrodeand the first ground electrode have widths approximately equal to thewidths of the waveguides beneath them.
 3. The optical modulatoraccording to claim 1, wherein the hot electrode and the first groundelectrode have substantially equal widths.
 4. The optical modulatoraccording to claim 1, wherein the hot electrode and the first groundelectrode each have a width less than that of the second groundelectrode and not exceeding 15 μm.
 5. The optical modulator according toclaim 4 in which the spacing between the first ground and hot electrodesis smaller than the spacing between the second ground and hotelectrodes.
 6. The optical modulator according to claim 1 wherein thesecond ground electrode has a width at least five times greater thanthat of the hot electrode.
 7. The optical modulator according to claim1, wherein the second ground electrode has a width at least ten timesgreater than that of the hot electrode.
 8. The optical modulatoraccording to claim 1, wherein the spacing between the first ground andhot electrodes is between 10 and 30 μm and the spacing between thesecond ground and hot electrodes is greater and between 20 and 80 μm. 9.The optical modulator according to claim 1, wherein the dielectricmaterial comprises silicon dioxide with a thickness between 0.4 and 1.5μm.
 10. The optical modulator according to claim 1, wherein theelectrodes comprise gold having a thickness between 15 and 50 μm.